How much power do you
need? If you're running a micro-ATX motherboard with embedded graphics and
audio, and a single 5400rpm IDE drive, that whimpy 150W power supply may be
enough. But if your rig pushes half a gig of RAM, dual procs, Ultra2 SCSI RAID,
and a GeForce 256, even a 300W supply may leave you bent.

Don't judge your power supply by overall wattage alone, look at the demands on
each separate line. Nonetheless, when looking at overall wattage, add the
requirements of every device in your rig, then multiply that number by 1.5
(because you should be taxing from 30 precent to 70 pecent of your power
supply's capacity). Some of your individual components may actually list their
specific power requirements, so use these numbers for more accurate
calculations. And remember, if you need to figure out wattage, multiply the
voltage by the amps.